In an image reversal that's increasingly common today on the big screen, Jake Gyllenhaal has transformed himself from the creepy cameraman Louis Bloom in Nightcrawler to a fierce welterweight boxer in Southpaw. His secret to the rapid change? To truly become boxer-fit.

"I lived in [the competitive fighting] world for six months," he says. Leading up to filming he took in a number of HBO fights, along with director Antoine Fuqua, paying visits to Vegas and Madison Square Garden in New York for the Miguel Cotto versus Sergio Martinez fight. (He says Cotto is his favorite fighter.) He also trained daily, like a fighter, for months, including some time at the [Floyd] Mayweather Boxing Club in Vegas, at at the Church Gym in New York.

The movie is now in the can and being edited, but Gyllenhaal doesn't plan to put down the mitts any time soon. "I will box for the rest of my life," he says. "It's an impossible form to master because it's like a language and every day you find new ways to use what you have. I'm nowhere close to mastering it," he says. "I could talk for hours about it. It's so wonderful for the mind and especially for youth, to understand how to express your physical power. I think especially, talking as a man, to know you are capable of a certain power and to know that you don't have to use it."